[EN[ In this study, five Pinus radiata ecotypes and a species hybrid (O4, Pinus radiata x Pinus attenuata) were analyzed to evaluate their drought response and to determine their tolerance, recovery capacity and hardening. O4 plants were included as tolerance models due to the high drought resistance of P. attenuata (Begley, 2001). In each ecotype the physiological changes produced along short and long stress cycles were studied, including the possible hormonal signals and their interconnections. To the analysis, different statistic tools such as regressions, ANCOVAs, MANOVAs and principal component analysis were used for an easier understanding of this complex plant response to water stress.
The water deficit provoked an alteration of hydric status and gas exchange parameters, an osmotic adjustment and an increase of cell wall elastic modulus. Plant growth was also affected.
Phytohormones played a main role in the plant water stress response, recovery capacity and hardening. Thus, stressed plants decreased their cytokinin levels, being the first water stress hormonal signal. The needle zeatin levels and hydraulic conductance decreased together, and when this reduction reached values of 65%, stressed plants started to accumulate indole-3-acetic acid (IAA) and abscisic acid (ABA). IAA was the most representative phytohormone in P. radiata plants subjected to sequential drought cycles. The needle IAA presence in stressed plants was mainly related to the external symptoms such as needle epinasty and apical curvature.
Drought also induced jasmonic acid accumulation and diminution of 1-aminocyclopropane-1-carboxilic acid (ACC), the ethylene precursor. These traits were mainly observed in ecotypes with high percentage of external symptoms. On the contrary, salicylic acid was accumulated in plants that presented low electrolyte leakage and less external symptoms.
Plants from O4 and O5 were the most drought tolerant ones, and their tolerance was due to an efficient control of hydraulic conductance and stomatal closure, and a high active osmotic adjustment (OA) (70%) induced by de novo osmolyte synthesis, that allowed the turgor maintenance. Among analyzed osmolytes, soluble carbohydrates strongly contributed to
OA, whereas free amino acids and free polyamines showed a modest contribution. However, stressed plants highly increased some specific amino acids and polyamines, suggesting an implication in plant water stress response. At this respect, - aminobutyric acid, proline (Pro) and glutamic acid (Glu) were accumulated in plants under water stress, and especially Glu and Pro after drought conditioning. The most tolerant ecotypes, O4 and O5, also increased their Pro content during hardening, being a good indicator of acclimation capacity. In addition, O5 showed the highest values of putrescine, spermidine and spermine.
High desiccation tolerance and hardening capacity were also associated to structural characteristics such as less substomatal chamber and higher xylem cell area, and a resin duct size increase under drought conditions.
Our study showed that drought tolerance in Pinus radiata varies at intraspecific level and is modulated by physiological and morphological changes and hormonal signals that are interconnected each other, playing IAA the principal role.